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GLASS INNOVATIONS: From Butterfly Wings to Solar Cells

Researchers are using the conformal evaporated film by rotation technique to capture microscopic biological surfaces in a thin coating of glass.

Standing before the equipment used for this technique, Pantano holds a piece of conventional doubly refracting glass, which requires the deposition of 15 or more alternating layers of two different materials with precisely controlled thicknesses. The CEFR technique yields doubly refracting characteristics through the deposition of only one coating that is nanostructured by the biotemplate (i.e., butterfly wings).

Enlarged
view of the surface of butterfly wings after the application of the coating
using CEFR.

Someday,
your car might have the metallic finish of an insect or the deep black of a
butterfly’s wing, and the reflectors might be patterned on the nanostructure of
a fly’s eyes, according to Penn State researchers who have developed a method
to rapidly and inexpensively copy biological surface structures. “Only a small
fraction of mutations in evolutionary processes are successful,” said Akhlesh
Lakhtakia, the Charles Godfrey Binder (endowed) professor of engineering
science and mechanics. “But evolution has gone on for at least a billion years.
A huge range of biological surface architectures have been created and are
available.”

Lakhtakia and his colleagues, Carlo G. Pantano, distinguished professor of
materials science and engineering, and director of Penn State’s Materials
Research Institute; and Raul J. Martín-Palma, visiting professor, Penn State,
and professor department of applied physics, Universidad Autonoma de Madrid,
used the conformal evaporated film by rotation (CEFR) technique to produce
coatings that capture the micro- and nanostructure of biological surfaces in a
thin coating of glass. The results appear in recent issues of Applied Physics
Letters
and Nanotechnology.

Duplicating Nature

In the CEFR technique, the researchers thermally evaporate
the material that forms the coating in a vacuum chamber. The object receiving
the coating is fixed to a holder and rotated about once every two seconds. The
researchers have coated butterfly wings and a fly, creating replicas of these
templates with identical surface characteristics. The researchers are using
chalcogenide glasses composed of varying combinations of germanium, antimony
and selenium.

“With the right temperature, which is room
temperature, and the right pressure and rotation speed, the coating process
takes about 10 minutes and deposits a 500-nanometer layer,” said Lakhtakia.

Some biostructures, such as moth’s eyes, which are duplicated to produce
moth’s-eye lenses, can be mechanically created by engineers, but it is
painstaking and expensive work. These lenses, which capture nearly all
available light, have applications in optoelectronic and photovoltaic
applications.

Other biostructures do not lend themselves
to synthetic reproduction. “In that case, perhaps we need to replicate the
actual structure,” said Lakhtakia. “One insect has an iridescent shell that
does not change colors as many shiny ones do. No one has made this type of
material artificially because we do not know the mechanism by which it retains
its color, but making a template from the actual insect would replicate the
fine structure of the surface.”

Many things in the natural world are colored
not by pigment but by surface structure. The way light interacts with the
surface creates the color, rather than any tint or chemical. Reproducing the
surface reproduces the color. Surface properties include not just visible light
characteristics, but also infrared, thermal, stickiness and other
characteristics.

The
magnified head of a fly coated with chalcogenic glass.

High-Tech Potential

Martín-Palma, Pantano and Lakhtakia’s work creates either a
replica template or a mold, depending on what they coat. The replica of a
template can be used to create a mold in a harder, less damageable material to
make many copies. Molds can be combined and multiplied to create the desired
surfaces.

The researchers initially looked at surfaces
with optical properties because they are easy to see and identify. The
structural black of some butterflies invites investigation of thermal
properties as well. Creating surfaces that have micro- or nanoscale patterns on
solar cells, heat exchangers, reflectors and lenses can produce devices that
work more efficiently.

“The whole world of biomimetics and
bioinspiration is just beginning to emerge,” said Martín-Palma. “Butterfly
wings come in a variety of surface structures. Eventually we may be able to
take these biological structures and modify them to create other properties
that do not already exist on biological surfaces.”

While the researchers are still experimenting
with butterfly wings, they would like to use CEFR on lotus leaves because they
are super-hydrophobic (surfaces that repel water could be very useful). They
also plan to look at other plant materials as potential surfaces for solar
cells.

Lakhtakia and Martín-Palma are organizing a small conference next year on
biomimetics and bioinspiration. Pantano suggested the use of chalcogenide glass
for its infrared properties, but the researchers have also tried other glasses
and materials like polymers to reproduce other surfaces and their properties.

This work was supported by the Ministerio de Educacion y Ciencia (Spain) and the
Penn State National Science Foundation National Nanotechnology Infrastructure
Network. The researchers have filed a provisional patent application on this
work.